bdiff: rename functions and structs to be amenable for later exporting

bdiff: use ssize_t in favor of Py_ssize_t in cpython-unaware locations This function and struct will be exposed via cffi, so we need to remove the cpython API dependency they currently have.

hgweb: enumerate lines in loop header, not before Doing this will allow access to the lines in arbitrary order (because the result of enumerate() is an iterator), and that will help calculating rowspan for annotate blocks.

sslutil: add assertion to prevent accidental CA usage on Windows Yuya suggested we add this check to ensure we don't accidentally try to load user-writable paths on Windows if we change the control flow of this function later.

shelve: make unshelve be able to abort in any case

osx: explicitly build hg with /usr/bin/python2.7 This should help avoid creating a package that depends on a custom Python, as happened when I built a package for 3.8.